genoksin electroplating of jewelry

7/27/2019 Genoksin electroplating of Jewelry

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Genoksin Electroplating of Jewelry

IntroductionElectroplating is a method to put a metal coating onto an object, in our case a piece of

jewellery, by placing it in a solution containing the metal to be plated and passing an electrical

current through the piece and the solution. It is possible to electroplate coatings of most pure

metals and even some alloys. In this paper, we shall concentrate on the electroplating of gold

and gold alloys and rhodiumone of the platinum group of metals with a good white colour

and tarnish resistancefor decorative applications. Electroplating is a comparatively quick

and easy process to carry out and does not require major investment in costly equipment. It

can be done successfully with very simple, basic equipment. Finished carat gold jewellery

may be electroplated with gold for several reasons:

1. electroplating carat gold jewellery with pure 24ct gold to impart a richer gold colour.2. electroplating with carat or pure gold to give a more uniform colour, hiding variations

in colour of the component parts and solder lines.

3. electroplating to give a different desired colour; a wide range of colour can beachieved by co-depositing gold with other metals.

4. electroplating to hide surface defects or to improve properties. In addition, there areother uses for gold electroplating, viz:

5. electroplating base metal or silver items with gold to obtain a gold appearance as infashion jewellery and gilt silver.

Rhodium is often used to give a good white colour to white gold jewellery (which is often not

a good white colour) or is used selectively on yellow gold jewellery to give local areas ofwhiteness, often around gem stone settings, and also to plate the master model made in silver

used for making the rubber mould in investment casting.

In contrast, electropolishing is the opposite to electroplating: we remove metal from the

surface of our jewellery by passing an electrical current in the opposite direction and, if we

get the conditions right, we can do this in a way that leaves a polished surface. Many

jewellery producers use electropolishing as part of their total finishing process in gold

jewellery manufacture. In this paper we shall discuss the following aspects:

Basic principles Factors in electroplating to be considered Surface preparation in electroplating Safety and pollution aspects Gold platingcaratage, colour, types of electrolytes and deposits Rhodium plating baths Equipment Electropolishing of gold jewellerytypes of electrolytes

Much of this information is contained in our two WGC publicationsThe Technical Manual

and the Finishing Handbook.


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Basic principles of electroplating and electropolishing

Electroplating and electropolishing is carried out in an electrolytic cell, Fig.1. This comprises

two electrodes that are electrically connected and immersed in a solutioncalled an

electrolyte. When an electrical current is passed through the cell, metal dissolved in the

electrolyte is deposited on the negative electrodethe Cathodewhilst the metal of the

positive electrodethe Anodemay be removed and dissolved in the electrolyte. Thus, metalpasses from the anode into solution in the electrolyte and is then deposited on the cathode.

Figure 1 - Schematic: electrochemical plating cell

Thus, if we make the cathode the piece of jewellery we wish to plate and the electrolyte

contains gold, then we can deposit gold on our jewellery item. On the other hand, if we make

our piece of jewellery the anode in a suitable electrolyte capable of dissolving gold, then,

under the right electrical conditions, we can remove the surface selectively to obtain a

polished surface. A typical electrolytic cell is shown in Fig. 2 and can enable several pieces tobe electroplated simultaneously.

Figure 2 - Typical electroplating bath

Often, in electroplating, we use an inert anode, where metal is not dissolved away, and control

the concentration of the depositing metal in the electrolyte solution by direct additions of the

appropriate metal as a salt to the electrolyte.


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The amount of metalin our case golddeposited is governed by Faradays law which says

that : The weight of metal deposited is proportional to the quantity of electricity passed.

The quantity of electricity is defined as the current (in amps) multiplied by the time (in hours).

The weight of metal deposited for a given quantity of electricity will be different for different

metals which is related to their atomic number and valency through a factor called theelectrochemical equivalent.

This Faraday law is very useful in calculating and controlling the amount (weight or

thickness) of metal deposited on a piece of jewellery. Obviously, at a constant electroplating

current (and salt concentration in the electrolyte), thickness of the electroplate is directly

proportional to the plating time. Double the plating time and you double the thickness.

Factors affecting the electroplating process

For decorative applications, we usually require a uniform thickness of electroplate over our

complex shaped item. This can be a problem at sharp edges and recessed surfaces, for

example. We usually also want a bright deposit, with good adhesion to the underlying item.We do not want the electroplated deposit to be highly stressed with a tendency to crack and

spall. We may want to plate at high speeds and still retain a good uniform bright surface. We

do not want a porous or micro-cracked coating which could allow corrosion or tarnishing of

the item during subsequent wear.

If we are co-depositing more than one metal, i.e. a carat gold, we also want good control of

compositiona uniform gold content over all the surface and throughout the thickness, for

example.

So how do we control these factors? Well, we achieve this through a number of ways:

Control of electrolyte formulation and pH (a measure of acidity or alkalinity) Control of anode surface area and position Control of electrical conditions Control of temperature

Firstly, the electrolyte. A good electrolyte will contain the metal (or metals) to be deposited in

solution in a sufficient concentration. In cyanide based gold baths, this will be in the form of

gold potassium cyanide salt. It will also contain other additives to give good plating

properties, These include, for example, additives to improve:

1. the throwing powerof the bath which means good uniformity of thickness over thepiece being plated.

2. the brightnessof the deposit. Special brighteners are added to assist.3. the internal stress in the deposit. These additives control the build-up of stress to

prevent cracking and spalling.

4. the chemical stability of the electrolyte and may include buffering agents to control pHwhich is a measure of the acidity or alkalinity of the electrolyte.

These additives are usually proprietary to the electroplating salt manufacturers and it is

difficult to find information on what they are. They are frequently organic chemical

compounds.


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During plating, it is usual to agitate or stir the electrolyte to maintain optimum plating

conditions and uniformity of composition.

The anode area and position are important to efficient electrodeposition and uniformity of

deposit. There is a tendency for plating to be thicker on cathode areas closest to the anode and

thinner in areas hidden (or out of line of sight) from the anode. Correct positioning of theanodes (more than one may be used) and a large anode area (compared to cathode area) is

desirable for good plating.

The electrical conditions during plating are also important for plating quality. In particular,

the current density (the current divided by surface area of the piece) plays an important role,

particularly in alloy plating where deposit composition is controlled by current density. If the

current is too high, the plating speed is increased but one may get a porous, dendritic deposit

rather than a bright one and it may be accompanied by gas evolution which affects the surface

finish. If it is very low, then the deposit may not have a good appearance and plating will be

slow.

The temperature of the electrolyte can also play a role in getting good plating, particularly in

alloy plating. Follow the electrolyte suppliers recommendations.

Surface preparation

For good quality electroplating and good adhesion of the deposit, the condition of the surface

to be plated is important. Most plating defects arise from unclean surfaces prior to plating.

The surface to be plated must be clean and free from grease, dirt, oxides and tarnish films,

polishing compounds, etc. Greasy, dirty surfaces will not be wetted by the electrolyte and may

not be plated. It also helps to have a smooth polished surface, free from defects and

imperfections, if one wants a bright polished electroplated deposit. Plating should not be used

to hide defects and to improve the surface polish (reduction in surface roughness). Defects to

be avoided include casting porosity, inclusions and embedded polishing compounds, scratches

and tool marks, and pitting from over-pickling.

The surface to be plated (the substrate) can be prepared by normal polishing techniques and

then cleaned in several ways:

Ultrasonic cleaning in detergent solution Degreasing in solvents, preferably in an ultrasonic bath Acid cleaning with pickling acids

Steam cleaning under a high pressure jet of steam Electrolytic cleaning; this can also activate the surface. Chemical cleaning with reagents, often at high temperatures.

In practice just one, or possibly two, techniques are used, for example degreasing and acid

pickling, followed by rinsing in water and drying. Many proprietary cleaners are alkaline with

wetting agents and surfactants added. Deionised or distilled water should be used as the final

rinse before drying to prevent any deposits from the water being left on the surface.

Safety and pollution aspects

Many electrolytes are based on cyanide. This is particularly true for gold. Cyanide is very

poisonous and must be handled with great care.


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A golden rule is never to allow drinking and eating in an electroplating facility and to have

very strict control and procedures in the plating shop. Protective overalls and visors should be

worn and changed regularly. Cleanliness is vital. For safety, cyanide electrolytes and plating

salts should be kept in locked cupboards. Keep cyanides and acids apart from each other. Acid

will react with cyanide to liberate deadly hydrogen cyanide gas!

Old electrolytes, as well as cleaners and rinse waters must be disposed of safely and NOT

thrown away down the sink or drain. The consequences of doing so are too awful to

contemplate!

Acid based, non-cyanide electrolytes must also be handled with care.

All reputable salt or electrolyte manufacturers will provide Materials Safety Data Sheets on

their products and give good advice on health and safety procedures

Gold electroplating systems

There are many electroplating systems on the market for putting pure gold and gold alloydeposits on to gold jewellery and on to base metals for decorative applications. There are also

many others for technical applications such as electrical contacts and connectors, where the

coating properties must have a certain technical performance.

The electrolytes can be classified into cyanide and non-cyanide based and may contain small

alloying additions to control colour and other properties. All cyanide-based electrolytes are

based on the use of gold potassium cyanide salt, KAu(CN)2, which contains about 68% gold.

However, most electrolytes do not contain anything like this concentration of gold. Some

electrolytes are acid, others neutral and others are alkaline, as shown in the classification in

Table 1.

Electrolyte type pHGold

complexAlloying metals

Alkaline 813 KAu(CN)2 Cu, Cd, Ag, Zn

Neutral 68 KAu(CN)2 Cu, Cd, Ag

Weakly acid 36 KAu(CN)2 Co, Ni, In, Fe

Acid0.5

2.5KAu(CN)4 Co, Ni, In, Sn

Cyanide-free,alkaline

8 - 10 Na3Au(SO3)2 Co, Ni, In, Sn

Table 1 - Electrolytes for gold alloy electroplating

The range of colours possible and bath and deposit characteristics of electroplating systems

from one well-known manufacturer are shown in Figures 3 - 5. Note the optimum bath

temperature is often above ambient. The gold concentration is quite lowabout 0.17.0 g/l

and the speed of plating ranges typically from about 10 - 75 mg/amp/min. The time to plate 1

micron thickness ranges from 315 mins.

Bath type: 1 2 3 4


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Gold content, g/l 8 - 108 12 - 16 12

Bath temperature 6070C 50C 7075C 50C

pH 6 -7 6 7.5 - 8 7

Plating

rate,m/min

0.10.6 0.5 0.624 0.10.2

Current density,

A/dm2

0.21.0 ca. 0.8 ca. 140 ca. 1.0

Additives As/Ti /Pb As No As,Ti or Pb

Salts/acids Citrate,

phosphate,

phosph. acid

Citrate, phosphate Phosphate,

phosph. acid

Phosphate

Deposit :

Purity, %gold 99.999.99 99.9 99.9 99.9Hardness, HV 70 - 90 250 70100 100

Colour Yellow Deep Yellow Yellow Yellow

Appearance Semi-matte Bright Semi-matte Bright

Application Electronics Elec. Contacts,

Decorative

Electronics Decorative

Table 2 - Fine gold electroplating baths

Table 2 (above), shows some pure gold plating baths based on gold potassium cyanide saltfrom another well known German manufacturer.

This illustrates the high purity of the deposit and how the properties of the deposit are

influenced by plating conditions and electrolyte composition. Note the high hardness values

compared to bulk pure gold.

For jewellery application, a deposit thickness of about 0.55.0 microns is typical, but very

thin flash coatings may be used where cost is more important than quality.

If one is gold plating onto base metals, it is common practice to first electroplate with a thin

flash or strike coat of copper to provide a good key , then an undercoat of nickel, bronze ortin. The purpose of these underlayers is to provide levelling and brightening to the substrate

and to inhibit migration of underlying copper into the gold layer, causing it to turn redder.

With the European Directive against use of nickel, there is a trend to use bronze (copper-tin

zinc) or tin or palladium as the underlayer.

Often, a strike gold layer is then deposited of about 0.1 microns thickness before the full

gold layer is electroplated from a different gold electrolyte. These are known as duplex

systems.

In selecting an electrolyte and plating system, it is good practice to seek advice from your

plating materials supplier. They can advise on what is most suitable for your needs. Plating, ofcourse, removes gold from the electrolyte. Therefore, it is important to maintain the correct


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concentration of salt in the electrolyte. Additions of salt should be made periodically. This

requires an ability to measure the gold concentration in the bath.

Between each stage of surface preparation and electroplating, it is important to rinse the items

being plated before moving to the next stage. This prevents contamination of the new bath

and loss of precious metal salt. This is known as drag-out. Of course, after completion of thetotal process, the item should be rinsed and dried. Do not use tap water as this will leave

deposits on the surface after drying.

Rhodium electroplating systems

Rhodium is a platinum group metal with a good white colour and is hard and tarnish resistant.

For jewellery purposes, we desire a bright deposit, defect-free and hard and there are several

suitable rhodium plating systems on the market. These are sulphate type baths and are very

acidic.

Usually, deposit thickness of about 0.5 microns, but up to 2-3 microns, is plated on gold

jewellery to give the required surface characteristics. There is a tendency for internal stress tobuild up in the deposit as thickness increases, resulting in cracking eventually.

For the high carat golds, a thicker layer of rhodium is plated directly on the substrate, but for

low carat golds, a nickel interlayer is plated first, allowing a thinner, cheaper rhodium deposit

without losing colour and providing good corrosion resistance.

As with gold, good surface preparation is required to provide a clean surface for quality

electroplating. The following practice is recommended:

Electrolytic cleaning Rinse in demineralised water Check surface wettability (no formation of droplets) Dip piece in aqueous solution of sodium cyanide (35 g/l) Rinse in running water or demineralised water Dip in electrolyte with power on (do not touch) & electroplate Rinse and dry.

Plating should take from 30 seconds to 2 minutes, depending on thickness desired. Inert

anodes of platinum are used at 4-5 cm distance with a surface area at least as big as the

cathode. The bath should be well agitated or stirred.

Periodic replenishment of the rhodium in the bath is necessary and this is done with special

rhodium replenishment solutions which have a high rhodium concentration and low acidity. It

is important to avoid contamination of the electrolyte by other metals, so good rinsing and use

of non-metallic tanks is recommended.

A typical rhodium electroplating system has the characteristics shown in Table 3.

The extremely high hardness of the deposit is notable. This is an advantage in rhodium plating

master models in silver for investment casting and electroforming as it enables a high degree

of polish to be obtained on the model, with benefit down the line to the casting or electroform.


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Electrolyte additives such as magnesium sulphate, selenic acid and sulphites are often used to

control internal stress build-up.

Table 3 - Typical bright rhodium electroplating system

Rhodium content 1.52.5 g/lBath temperature 4050C

pH < 1

Plating rate 2 mg/A/min

Current density 1.55.0 A/dm2

Salts/acids Sulphuric acid

Deposit purity, rhodium % 99.9

Time to deposit 1 micron 30 secs.

Deposit appearance Bright

Hardness of deposit HV 950

Electroplating equipment

Basically, electroplating is a simple process and can be performed in simple glass beakers

with a simple d.c. electrical supply. However, if good consistent quality is desired, it is

preferable to use purpose-made equipment, which will include:

Plating tankpreferably in glass or plastic, with lid (e.g. pyrex glass, Teflon,Polypropylene, PVC, HDPE)

Additional rinsing tanks, with lids A reliable D.C. power supply with sufficient current output Electrolyte temperature heating and control system Stirrer, pump and filtration systems Insoluble anodes (often platinum or platinum-plated titanium sheet or gauze) Inert connecting wires for electrodes where immersed in bath Ability to plate several items simultaneously Fume cupboard or fume extraction.

Whether one is only plating on a small scale on a bench or on a mass production scale, there

are many suppliers of purposemade equipment to suit all needs. Some examples are shown inFigure 6. They can often be viewed at the major jewellery shows, e.g. at Basel and Vicenza in

Europe. Prices do vary significantly, but it is possible to buy suitable equipment quite

cheaply, or even to find a local fabricator to make one tailored to your needs.


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Figure 6 - Typical commercial electroplating equipment, for

small bench operations (a) range of sizes (b) in use

It is appropriate to make a comment on masking of surfaces so that electroplating is only done

in areas where it is wanted, e.g. around gem stone settings. This is done by painting on an

organic lacquer (often pink in colour) to those areas where plating is not wanted and allowing

it to dry. After plating, it can be easily removed with an organic solvent such as acetone.

There are many commercial products on the market.

Remember that such lacquers are inflammable and must be stored in well closed containers.

More details of masking lacquers are given in the WGC publication, The Finishing

Handbook.

Electropolishing of gold jewellery

The equipment for electropolishing is very similar to that for electroplating as seen in the

sketch, Figure 7, and is manufactured by the same companies. The cathode is normally

stainless steel or titanium as is the anode frame, which has platinum suspension wires or

hooks. This anode frame may need to be agitated. Again the bath is heated, in this case by an

immersion heater, and there is fume extraction. A D.C. power supply supplying low voltage

(6- 15 V) and a high current is needed to give a current density in the range 100150 A/dm2.

Typical bath temperature is up to 80C and a system for stirring the electrolyte is also

necessary.


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Figure 7 - Schematic:

electropolishing cell

To explain how electropolishing is achieved, it is necessary to examine the anode polarisationcurve which plots current density against applied voltage, Figure 8. Such curves are

characteristic for each electrolyte and metal item. If we operate the electrolytic cell at the lowvoltage portion of the curve AB, nothing much happens to our jewellery. At higher

voltages, in the region B - C, etching of the surface occurs and this will reveal details of the

metallographic structure of the surface under the microscope. In the region DE, the current

density remains constant, despite increasing voltage. This is the range where good

electropolishing takes place. This is where we operate the process! At voltages higher than E,

the current density increases rapidly and there is gas evolution at both cathode and anode

which is undesirable for a good polished surface.


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Figure 8 - Electropolishing: anode polarisation curve

The mechanism of electropolishing is complex and it is not appropriate to discuss it here.

However, the rough surface is levelled in the process and a good bright smooth surface can be

achieved, as shown in the examples in Figure 9.

Figure 9 - a) Electropolished jewellery: as cast 14 ct gold

(right) andafter electropolishing (left)


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There are many factors which influence the process, including jewellery alloy, electrolyte

composition, temperature, current density & voltage and time.

There are several proprietary electropolishing systems on the market for electropolishing gold

alloys in the range 8ct up to 24ct, many using the safer, cyanide-free, weakly acid electrolytes,

operating at temperatures up to 80C. The older systems use cyanide-based electrolytes,operating at 8090C. As with electroplating, it is important to rinse the jewellery after

electropolishing and to dry it.

Clearly, gold is dissolved from the surface in the process. This is small if the initial surface is

good. Chains and all types of jewellery can be electropolished. The process does not discolour

the jewellery, even at solder lines. Good rinsing and the use of a brightening chemical

solution are recommended after electropolishing.

The gold that is dissolved in the electrolyte from electropolishing can be recovered. For

cyanide-freesolutions, the electrolyte is treated with sodium hydroxide until a pH of 5 is

attained. Then a special reducing compound is added and gold is precipitated from solution. Itis allowed to settle and filtered off. More sodium hydroxide is added to the remaining solution

until pH 5-7 is reached and then safely disposed of down the drain. The gold slime filtered off

is dried, mixed with borax flux and melted. It is poured off and allowed to solidify into a

small bar or button. For cyanide solutions, the gold can be precipitated by additions of zinc or

aluminium dust.

Electropolishing of gold jewellery can be done as a single finishing step but, more often, it is

part of a multistep process involving mechanical polishing as well.

The advantages of electropolishing are:

It is quick It can polish complex shaped items and contours will be preserved Recovery of dissolved gold is easy Its disadvantages are: Only metal is removed from the surface. Defects such as casting porosity are made

more evident! This can be advantageous in identifying faulty jewellery.

It will remove very small defects (1-2 microns) but not larger defects.Concluding remarks

ElectroplatingWe have discussed the basic principles of electroplating and some of the factors affecting the

process. We have also discussed the equipment requirements.

As we have seen, electroplating of jewellery is a very versatile process and one can obtain

gold coatings of varying colour, appearance, properties and caratage as well as pure gold. It is

a quick, cheap and easy process to operate.

It does not require expensive equipment, but it is worthwhile to buy good quality

electroplating salts from reputable suppliers. Such salts are specially formulated to give good

performance.


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Many gold plating processes use toxic cyanide electrolytes. Care must be taken in their use

and disposal.

genoksin electroplating of jewelry

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